Analysis of the free amino of Fasciola hepatica extracted from the worm with 70% ethanol reveals that between 25-35% of the total is proline. Analysis of the free amino acids in the bile fluid of rats infected with Fasciola hepatica shows a very marked increase in the level of proline, beginning with about a 100 fold rise 25 days after infection and increasing to about 3000-5000 times after the worms enter the ducts which occurs about the 70th day. There is also an increase in the concentrations of the other free amino acids. However, the increase in any of these amino acids is less than 10 fold and is detectable only after the worms have entered the bile duct, while proline increases over 100 fold 6 to 7 weeks before the worms enter the bile ducts. At the time this increase is detected there is a concomitant increase in the bile duct epithelium of the host resulting in a thickened, extensively folded, epithelium and a dilated duct, in preparation, as it were, for he living site of Fasciola hepatica. It has been determined that the worm is the most likely source of the excessive proline in bile. This is concluded from studies on homogentes of Fasciola of two enzymes involved in the metabolism of proline: ornithine -delta- transaminase and proline oxidase. Ornithine-delta-transaminase which forms the immediate precursor of proline has 7-10X times the activity found in homogenates of mammalian liver. Proline oxidase, the only enzyme which breaks down proline is present in very low concentrations or not at all in fluke homogenates. Studies planned on the enzymatic apparatus of Fasciola with regard to the biosynthesis of proline as a possible end product of nitrogen metabolism and/or as a hydrogen sink will be a significance in attempting control of this and other trematodes in the host. Furthermore, if the agent causing such drastic alterations of normal bile duct epithelium can be derived from the parasite, isolated, and characterized, it could be of vital significance to the widespread areas of host-parasite integration, biochemical differentiation and perhaps cancer research.